Conventionally, there has been a technique for using a voltage variable power supply as the power supply of an amplifier to follow the level of an input audio signal so the amplifier in order to increase or decrease the level of supply voltage supplied to a power amplifier stage, thereby reducing noise superimposed on an output signal and improving the power efficiency of the power supply.
Thus, the supply voltage supplied to the power amplifier stage is made to follow the input audio signal level. When the input audio signal level is low, since the supply voltage at the power amplifier stage can be reduced to a voltage level with an amplitude that does not distort the amplified signal, the noise superimposed on the output signal of the amplifier can be reduced and the power efficiency of the power supply can be improved.
Further, the output signal is negatively fed back to a signal input part so that the noise or distortion superimposed on the output signal can be corrected.
In this case, when the supply voltage supplied to the power amplifier stage is reduced, the amplification gain of the amplifier clue to voltage amplification at the power amplifier stage also drops. Therefore, there is known a technique for adjusting the gain of gain adjustment means provided in a feedback loop according to an increase or decrease in the level of voltage supplied to a switching amplifier of a signal reproducer to adjust the gain of a negative feedback loop to keep the gain constant (see Patent Document 1).